Electro-fusion joints have been developed recently for use as pipe joints and saddle joints for the purpose of connecting plastic pipes such as running water service pipes and gas pipes.
In general, an electro-fusion joint is a molded part having a coil embedded therein. In use, pipes to be connected together are inserted into both ends of the joint and electric power is supplied to the coil so that the surfaces of the joint and the surfaces of the pipe contacting each other are heated and fused together whereby the pipes are connected to each other by means of the joint.
The electrical power supplied to the joint largely affects the quality of the joint. For instance, a too large electrical power supplied to the joint tends to cause a deformation of the joint portion, particularly the plastic pipes. Conversely, a too small electrical power produces only small amount of heat, with the result that the bonding strength is lowered undesirably. The device for supplying electrical power to the joint is usually controlled manually. Namely, the operator increases or decreases the amount of electrical power upon judging the state of fusion from the appearance of the joint.
Since the control of the electrical power is conducted manually, the quality of the connection largely depends on the degree of skill and experience of the operator, so that the quality tends to fluctuate undesirably.
Under this circumstance, a method has been proposed and used in recent years in which a plurality of levels of electrical power are beforehand set corresponding to sizes and types of the electro-fusion joints and the operator selects one of these set levels after visual recognition of the size and the type of the joint which is to be used, so that the connection is conducted with the electrical power of the set level.
A method also has been proposed in which an identification resistance of a value corresponding to the type of the joint is embedded in the pipe joint in addition to the heat-generating coil for fusing the joint such that the type of the joint can be identified through detection of the resistance. Then, the connecting operation is conducted by supplying electrical power which has been set in relation to the level of the identification resistance.
In a similar method which also has been proposed, the type of the joint is identified through measurement of the resistance value of the heat-generating coil in the joint, and the electrical power which has been set in correspondence with the joint type is supplied.
The selection of the set levels of the electrical power upon visual recognition of the joint size and type requires a laborious work for checking up the data showing the relationship between the size and type of the joint and the level of the electrical power.
The method which identifies the type of the joint through measurement of the resistance of the heat-generating wire enables the user to identify the type of the joint simply by inserting a plug, jack or a connector on the end of a cord and the electrical power of the level corresponding to the identified joint type is automatically supplied.
However, the method which relies upon the recognition of the joint type through detection of the identification resistance separate from the heat-generating wire also is disadvantageous in that the construction is complicated to raise the cost of the joint.
This problem is overcome by the method which makes use of detection of the resistance value of the embedded heat-generating wire. In this method, however, the freedom of design of the joint is undesirably restricted because the heat-generating wire is required to have the function for identifying the type of the joint.
Namely, the resistance value varies depending on the resin material of the resin. In addition, designing of heat-generating wires for optimizing heating conditions for various sizes and types of joint may cause overlapping of resistance value of the heat-generating wire over different sizes and types of joint. Conversely, if the design is made while giving a priority to the identification function, the optimum heating condition may not be attained.